1. Field of the Invention
This invention relates generally to a process for carrying out heterogeneously catalyzed hydrogenation reactions, and more particularly to heterogeneously catalyzed hydrogenation reactions in a fixed-bed reactor for hydrogenating fatty compounds, including native fats, oils, and derivatives thereof, including fatty acid methyl esters to form saturated fatty alcohols.
2. Background Information
The heterogeneously catalyzed hydrogenation of fatty acid methyl esters (FAME) to saturated fatty alcohols in fixed-bed reactors has the particular asset that an almost complete conversion of the educt to the required product is basically possible without having to accept serious losses of selectivity through consecutive hydrogenation to hydrocarbons or through unwanted secondary reactions.
In practice, however, the heterogeneous catalyst (for example Cu—Cr) undergoes significant deactivation in the course of a production campaign which may be attributed to the following influences:    1. Deactivation of the active catalytic surface by compounds of S, Cl and P as discussed by Twigg, M. V. et al., Deactivation of copper metal catalysts for methanol decomposition, methanol steam reforming and methanol synthesis, Topics in Catalysis, Vol. 22, No. 3-4, 2003, and Appl. Catal., Vol. 212, pp. 161-174, 2001;    2. Sintering of the active Cu surface by high local temperatures, as discussed by Twigg, cited above, and in “Methanol Synthesis”, Hansen, J. B., Handbook of heterogeneous catalysis, Vol. 4, pp. 1859-1860, edited by Ertl, G., Knötzinger, H. and Weitkamp, J., VCH, Weinheim, 1997;    3. Discharge of the active species through formation of copper soaps and attacking of the matrix through the formation of zinc soaps in the case of Cu—Zn catalysts and a high content of free fatty acids as discussed by Rieke, R. D. et al., FAME hydrogenation to fatty alcohol Part I: Correlation between catalyst properties and activity/selectivity, JAOCS, Vol. 74, No. 4, pp. 333-339, 1997, and Schneider, M. et al., Die Anwendung von Kupferchromit-Katalysatoren in der Hydrierung von Fettsäureestem zu Fettalkoholen, Feft Wissenschaft Technologie, Vol. 89, S. 508-512, 1987;    4. Reversible deactivation, for example through traces of free glycerol, monoglycerides and water as discussed by Thakur, D. S., et al., Fatty Methyl Ester Hydrogenation to Fatty Alcohol: Reaction Inhibition by Glycerine and Monoglyceride, JAOCS, Vol. 76, No. 8, pp. 995-1000, 1999.
The following publication may also be of interest: Voeste, T. et al., Production of fatty alcohols from fatty acids, JAOCS, Vol. 61, No. 2, p. 350-352, 1984.
The average life of the fixed-bed catalyst in the industrial-scale production of fatty alcohols is around 60 to 120 days as discussed in Gritz, E., Fat hydrogenation, Handbook of heterogeneous catalysis, Vol. 5, pp. 2221-2231; edited by Ertl, G.; Knödtzinger, H. and Weitkamp, J., VCH, Weinheim, 1997. Sintering of the catalyst can be avoided on the one hand by activating the catalyst at moderate temperatures (for example, at most 200° C.) as discussed in Methanol Synthesis, cited above, so that deactivation through sintering during the production period is significantly limited. In addition, thorough dispersion of the catalyst leads to less sintering. On the other hand, ageing through raw material impurities, i.e., catalyst poisons, can be avoided by using only freshly prepared, i.e., purified, FAME fractions.
In practice, gradual ageing of the catalyst is still observed over several months, as known from Gritz, cited above. The loss of activity of the catalyst can be compensated by the following measures in order to maintain high conversions for almost undiminished selectivity:    1. raising the temperature at the reactor entrance or in the reactor casing,    2. reducing the throughput of liquid.
Raising the temperature is limited for two reasons. First, a high temperature promotes the formation of hydrocarbons of which the maximum concentration in the marketable product must not be exceeded. Second, raising the average temperature involves the danger of the activity of the catalyst being additionally reduced by increased sintering. There are also limits to the second measure insofar as any reduction in the LHSV (liquid hourly space velocity) results in a direct reduction in the volume/time yield. In addition, there is generally a deterioration in wetting, so that, with lower throughputs, the reactor is no longer operated at optimal efficiency.
The time at which the catalyst bed is renewed is thus determined by the basic economic conditions. In general, the production cycle may be divided up into the following phases:    1. Charging;    2. Activating the catalyst bed;    3. Startup;    4. Production under optimal operating conditions;    5. Reducing the throughput to compensate for catalyst deactivation;    6. Raising the temperature as further compensation for ageing of the catalyst; and    7. Shutting down the plant and emptying the reactor.
In order to increase the life of the catalyst in the reactor, it is known that an auxiliary reactor (guard bed) can be installed in front of the reactor as described by Twigg, cited above. Accordingly, the starting product is passed first through the auxiliary reactor, which traps a large proportion of the constituents harmful to the reactor (catalyst poisons), and then into the main reactor. Since the concentration of fatty acid methyl ester is maximal in this auxiliary reactor, so that the reaction rate is particularly high, heat is generated in particular abundance by the exothermic reaction, so that sintering is particularly high in the auxiliary reactor. Accordingly, the auxiliary reactor also protects the main reactor against sintering.
However, the disadvantage of the prior art is that the catalyst in the auxiliary reactor is consumed particularly quickly. To change the catalyst, the plant firstly either has to be shut down or, secondly, the starting product has again to be passed directly—and disadvantageously—through the main reactor, meaning that the unwanted increased deactivation of the catalyst in the main reactor has to be accepted. In addition, throughput has to be reduced in this case on account of the reduced total amount of catalyst because substantially complete reaction of the fatty acid methyl ester to fatty alcohol is necessary in order to avoid subsequent difficulties during working up of the reaction mixture. Thirdly, the auxiliary reactor could basically be replaced by a second auxiliary reactor with a fresh catalyst bed. The disadvantage of this would be that only half the total capacity of the two auxiliary reactors would be utilized because only one auxiliary reactor would ever be in operation.